Polycarbonate, polymethyl methacrylate and other thermoplastic resin substrates are generally characterized by a number of superior properties including transparency, ductility, high heat distortion temperature, and dimensional stability. Most of these materials are transparent and have long been used as the replacement for glass in many commercial applications. However, these materials may lose transparency because they are readily damaged by scratching and abrasion. Additionally they are susceptible to UV degradation. There thus occur undesired phenomena like substrate yellowing and surface whitening. As one solution to such drawbacks, resin substrates are surface covered with mar resistant coatings having UV absorbers compounded therein.
The mar resistant coatings having UV absorbers compounded therein, however, suffer from the problems that the coatings fail to exert their performance to the full extent because of bleed-out or leaching-out of UV absorbers, and their mar resistance is degraded by compounding of UV absorbers. One prior art approach to overcome these problems is by fixedly binding the UV absorber to the binder component in the mar resistant coating for thereby suppressing any losses of weather resistance and mar resistance due to bleed-out and leaching out. In this approach, the UV absorber must be designed in accordance with the main chain structure and crosslinking mode of the mar resistant coating binder.
For instance, a silicone hard-coat composition comprising a hydrolytic condensate (or precursor) of alkoxysilane has satisfactory mar resistance and durability. The composition is cured via crosslinking reaction by condensation of silanol (SiOH) on the precursor, forming a cured coating. In this silicone hard-coat composition, the UV absorber having a reactive group capable of silanol crosslinking is incorporated, examples of which are known from Patent Documents 1 to 4. However, these reactive silylated UV absorbers suffer from process problems including multiple stages in their synthesis route and removal of hydrosilylation catalyst. In general, a silicone hard-coat film must be laid on a resin substrate via a primer before a tight bond can be achieved between the film and the substrate. Then additional steps including preparation, coating and curing of the primer are necessary.
Another known example of the mar resistant coating is a photo-curable (meth)acrylic coating composition comprising a multifunctional (meth)acrylate and a photopolymerization initiator. Photopolymerization of (meth)acrylic groups in the multifunctional (meth)acrylate induces crosslinking, forming a cured film. In this photo-curable (meth)acrylic coating composition, the UV absorber having a reactive group is incorporated, examples of which are known from Patent Documents 5 to 9. The photo-curable (meth)acrylic coating compositions comprising UV absorbers having a reactive group can be coated and cured to the resin substrates directly without a need for the primer which is essential for the above silicone hard-coat compositions, but still suffer from some outstanding problems.
Patent Documents 5 and 6 disclose alkoxysilyl-containing dibenzoylresorcinol derivatives as the UV absorber having a reactive group. It is believed that cured films have somewhat low mar resistance because the alkoxysilyl group does not participate in crosslinking reaction of photo-curable (meth)acrylic coating. In fact, Patent Documents 5 and 6 refer nowhere to mar resistance. A bleed-out risk is left during long-term weathering, because it is believed that the UV absorbers have not reacted with the binders.
Patent Documents 7 and 8 disclose silsesquioxanes having a (meth)acrylic functional benzotriazole UV-absorbing group. Although these UV absorbers are improved in solubility in multifunctional (meth)acrylate over the unmodified benzotriazole type UV absorbers, they include relatively much portions that do not contribute to UV absorption, like silsesquioxane skeleton. On the assumption that the term “UV-absorbing group content” of a compound refers to a percentage of UV-absorbing groups based on the molecular weight of that compound, these UV absorbers have a low UV-absorbing group content. To gain the necessary UV-absorbing capacity, the amount of the UV absorber loaded is inevitably increased, detracting from certain properties of cured coatings (other than weather resistance) such as mar resistance and adhesion.
Patent Document 9 describes a photo-curable (meth)acrylic polymer having a UV-absorbing group and a (meth)acryloyl group on side chains. This (meth)acrylic polymer reacts with a multifunctional (meth)acrylate in a photo-curable (meth)acrylic coating composition whereby it is fixed within the cured coating. This UV absorber has improved solubility in multifunctional (meth)acrylate, but a low UV-absorbing group content because of polymer form. This suggests that the amount of the absorber loaded must be increased, detracting from mar resistance and adhesion.
On the other hand, (meth)acrylic functional UV absorbers having a relatively low molecular weight and a high UV-absorbing group content are also known. For example, 2-[2′-hydroxy-5′-(methacryloyloxyethyl)phenyl]-2H-benzotriazole is commercially available under the trade name RUVA-93 from Otsuka Chemical Co., Ltd. Also, Patent Documents 10 and 11 describe benzotriazole and benzophenone UV absorbers having an unsaturated double bond linked thereto by a urethane bond via an alkylene chain. Further, benzotriazine UV absorbers having (meth)acrylic functionality bonded thereto are disclosed in Patent Documents 12 to 14. These (meth)acrylic functional UV absorbers of relatively low molecular weight are suitable for the synthesis of (meth)acrylic polymers as in Patent Document 9 because they have only one (meth)acrylic group per molecule, but unfavorable as the UV absorber in photo-curable (meth)acrylic coating compositions because the crosslinking density is reduced, detracting from mar resistance.
In general, UV absorbers of benzophenone and resorcinol type are insufficient in UV absorbing capacity. Some UV absorbers of benzotriazole type are known to be toxic. Those UV absorbers of benzotriazole type devoid of the safety problem are less soluble so that their loading is limited.
In connection with photo-curable (meth)acrylic coating compositions which can be directly applied to resin substrates without a need for primers, (meth)acrylic functional UV absorbers which has both a high solubility in multifunctional (meth)acrylate and a high UV-absorbing group content, and which do not detract from mar resistance and adhesion of cured coatings are unknown.